"And from my point of view, there must be a culture of openness so that anybody who knows about safety problems in hospitals feels able to speak out. We should encourage staff to speak out and not keep them quiet."

Figures obtained under the Freedom of Information Act show that at least 52 staff have been silenced using legal orders since 2008, some of which cost as much as £500,000. All are thought to contain confidentiality clauses.

The true number is likely to be higher. A quarter of hospital trusts in England and Wales failed to respond, while the survey did not include GPs, the ambulance service and mental health trusts.

The figures were obtained by Stephen Barclay, a Conservative member of the public accounts committee. He will today call for Sir David, the chief executive of NHS England, to step down immediately.

The NHS chief has been heavily criticised for his role in the scandal at Stafford hospital between 2005 and 2008, when 1,200 patients died after being routinely neglected.

Sir David was head of the West Midlands Strategic Health Authority, which supervised the trust at the centre of the scandal, before becoming chief executive of the NHS in 2007. He became chief executive of NHS England, a new arms-length body responsible for services, when it was formed in April. He said that he will take retirement next March.

A criminal inquiry into Mid Staffs is under way, and Jeremy Hunt, the Health Secretary, told Parliament that “no one is above the law of the land”. On Monday, Staffordshire Police said that up to 300 cases could constitute criminal neglect.

Mr Barclay said: “It is simply not plausible that the man who was supposed to be running the NHS was seemingly unaware that employees threatening to speak out were being offered golden goodbyes in return for a vow of silence.

“As the accounting officer who has presided over this culture he is either complicit in a systemic cover-up or has failed to ask questions. If he knew about them he has misled Parliament. The culture in the NHS needs to change, he has to stand down now. What patient safety concerns have been covered up [by these gagging orders]? How many lives have been put at risk?”

In March, it emerged that NHS hospital trusts had spent £15million on silencing almost 600 staff. But the figures did not include “judicially mediated” settlements, under which a hospital reaches a settlement with staff which is then signed off by a judge or senior lawyer.

Unlike normal gagging orders, the pay-offs are not signed off by the Department of Health or the Treasury, meaning that the Government has no chance to block them if they are inappropriate.

The Department of Health said it has now closed the “loophole”, but said that previous agreements will not be reviewed.

The biggest judicially mediated settlement was signed by Gary Walker, the former chief executive of the United Lincolnshire Hospitals Trust. He was removed from his post and paid £500,000 to keep quiet after raising concerns about patient safety.

Mr Walker, who was threatened with legal action when he went public with his concerns, said: “They [the agreements] are the most blatant sweeping under the carpet I have ever seen. They are sinister. I know a couple of people who have signed these agreements after raising patient safety concerns but are too afraid to speak out.”

When Sir David appeared before the public accounts committee in March, he claimed Mr Walker had not identified himself as a whistle-blower. He was subsequently forced to correct his evidence after it emerged that Mr Walker wrote to him four years ago raising concerns.

He also claimed at the committee that Mr Walker’s judicially mediated settlement was the “only one that I had ever come across”. He pledged to investigate the issue, but did not respond to Mr Barclay’s repeated requests for updates.

The figures show that two consultants at Northern Lincolnshire and Goole Hospitals NHS Trust, which is being investigated over high death rates, signed gagging orders worth £265,000 and £300,000 a month before Sir David’s appearance before MPs. Twenty-nine other trusts have signed judicially mediated settlements including Heart of England foundation trust, Royal United Hospital Bath trust, University Hospital Southampton foundation trust and County Durham and Darlington trust.

A spokesman for the Royal United Hospital Bath trust said its agreements did not contain confidentiality clauses and as such could not be considered to be "gagging orders".

A spokesman for NHS England said that the Department of Health was taking the lead on the issue. She said that Sir David did not know how widespread the orders were before the hearing, and that he said that he would contact all NHS bodies as soon as he possibly could.

She added: “Closing the loophole which meant that these payments did not need to be signed off by the Department of Health or the Treasury became a higher priority than writing out to find how many of these payments had been made. By the time this process was completed Sir David had taken up his role at NHS England and was no longer in a position to be able to write to the NHS on this matter.”

A spokesman for the Department of Health said: “The department did not collect data on these payments prior to February 2013. This has now changed: all non-contractual severance payments, whether via judicial mediation or another means, need to be scrutinised by a national body and they will not be recommended for Treasury approval unless the NHS can show that they have made staff fully aware of their legal right to blow the whistle.”

“Judicial mediation payments cannot prevent staff from speaking out about matters on patient safety or in the public interest.”